1. Field of the Invention
The present invention relates to a method of establishing a conversion relationship for predicting color reproducibility characteristics of color images that are printed.
2. Description of the Related Art
Printing machines or color printers generally generate color images by processing device data such as halftone dot % data of C, M, Y, K or density data of R, G, B for area modulation or density modulation on image support mediums.
Color reproducibility characteristics of color images that are printed on image support mediums depend on output conditions including color materials used, medium properties, etc. Even when only a particular output condition is varied, it is not an easy task to predict colors which will finally be reproduced on an image support medium, i.e., to predict color reproducibility characteristics of a final color image.
It has heretofore been customary to output a color chart comprising a plurality of patches on an image support medium based on known device data, measure calorimetric values of the patches, and thereafter carry out interpolating calculations to determine a color-predicting conversion relationship which is a relationship between device data with respect to particular output conditions and colorimetric values of a finally finished color image, i.e., a color-predicting function (mapping function) or a color-predicting lookup table (mapping lookup table) for converting C, M, Y, K or R, G, B values into colorimetric values. For determining a highly accurate color-predicting conversion relationship, it is necessary to output as many patches as possible and measure calorimetric values of those patches. However, since such a procedure must be effected for each of various combinations of output conditions, the conventional process of determining a highly accurate color-predicting conversion relationship has been highly time-consuming and tedious.
To eliminate the above drawbacks, there has been proposed a process for determining a correlation between halftone dot % values of an arbitrary patch and their colorimetric values by optimization of explanatory variables of multiple regression equations (see Japanese laid-open patent publication No. 4-337965). The proposed process requires the tasks of generating a considerable number of patches, measure colorimetric data of the patches, and generate the function, and hence is not necessarily so efficient as to solve the above drawbacks. It is practically infeasible to determine in advance a color-predicting conversion relationship with respect to all output conditions because there are many output conditions to be taken into account when prints are produced.
According to other proposed processes, a color-predicting function is defined by a theoretical formula which physically simulates basic output conditions in order to reduce efforts needed to output and colorimetrically measure patches (see Japanese patent publication No. 54-39762 and Japanese laid-open patent publications Nos. 2-11356 and 3-202978). These processes are tedious and time-consuming because it is necessary to determine various parameters of the color-predicting function by way of elaborate experimentation. In addition, calculations based on the color-predicting function are complicated.